If a model is attacked every model attacking the same enemy model get this bonus.
Even if it is an attack using the same action token spent (an attack which happens simultaneously).

This rule represents that these models are trained to fight together and for that reoson the 2nd and further models attacking the same enemy model gain a bonus.

If you spend more action tokens to initiate another close combat even the first attacker will get a bonus against models which got attacked before...

Example:

Numbers= runts, letters are enemies
12 against A
34 against B
56 against C
1,3,5 get no bonus, 2,4,6 get a +1 bonus
If the runts spend more Action Tokens to attack again, all runts (1-6) get the bonus.